Jusung
An†
a,
Hyunsik
Hong†
b,
Miae
Won
a,
Hyeonji
Rha
a,
Qihang
Ding
a,
Nayeon
Kang
b,
Heemin
Kang
*b and
Jong Seung
Kim
*a
aDepartment of Chemistry, Korea University, Seoul 02841, Korea. E-mail: jongskim@korea.ac.kr
bDepartment of Materials Science and Engineering, Korea University, Seoul 02841, Korea. E-mail: heeminkang@korea.ac.kr
First published on 13th December 2022
Mechanical stimulation utilizing deep tissue-penetrating and focusable energy sources, such as ultrasound and magnetic fields, is regarded as an emerging patient-friendly and effective therapeutic strategy to overcome the limitations of conventional cancer therapies based on fundamental external stimuli such as light, heat, electricity, radiation, or microwaves. Recent efforts have suggested that mechanical stimuli-driven cancer therapy (henceforth referred to as “mechanical cancer therapy”) could provide a direct therapeutic effect and intelligent control to augment other anti-cancer systems as a synergistic combinational cancer treatment. This review article highlights the latest advances in mechanical cancer therapy to present a novel perspective on the fundamental principles of ultrasound- and magnetic field-mediated mechanical forces, including compression, tension, shear force, and torque, that can be generated in a cellular microenvironment using mechanical stimuli-activated functional materials. Additionally, this article will shed light on mechanical cancer therapy and inspire future research to pursue the development of ultrasound- and magnetic-field-activated materials and their applications in this field.
Nayeon Kang | Nayeon Kang received her BS degree in Materials Science and Engineering from Korea University, Seoul, in 2021. She is pursuing her PhD degree in Prof. Heemin Kang's laboratory since 2021. |
Key learning points1. General significance of mechanical stimuli in cancer therapy.2. Fundamental mechanism of mechanical stimuli-driven cellular signaling. 3. The clinical advantages of mechanical stimuli-driven cancer therapy mode using tissue-penetrative ultrasound and magnetic fields. 4. The strategies for enhancing mechanical stimuli-driven cancer therapeutic efficacy. 5. Recent development, challenges, and perspectives for mechanical stimuli-driven cancer therapy strategies. |
The fundamental mechanical forces are intrinsically present throughout the cellular microenvironment due to normal motion and physiological functioning of cells and sub-organelles.15 For several decades, studies have revealed that mechanical forces drive numerous physiological processes and are a crucial regulator of cellular interaction.16,17 Conversely, a cytotoxic factor can be produced within the cell environment when the intensity of exogenous mechanically forced stimulation achieves an excessive level that can directly damage the cell.7,8,17 This level of mechanical force can contribute to creating appropriate conditions for enhanced cancer therapy by generating a structural change that can directly affect cancer cells.17 Thus, the last decade has witnessed tremendous progress regarding the use of “mechanical stimuli” in the cancer treatment field based on a new angle of utilizing ultrasound and magnetic fields. Most recently, as Ardem Patapoutian won the Nobel Prize in Physiology or Medicine in 2021, it is valid to consider the established theory that mechanical activation directly affects the biological environment.18 The mechanical cancer therapy driven by ultrasound and magnetic fields is based on the fundamental mechanical forces with the subclassification of compression, tension, shear force, and torque that are ubiquitously achieved in a cellular microenvironment or easily applied externally. To better understand the role of microscopic mechanical stress in the context of cellular morphogenesis caused by ultrasound and magnetic fields, the physical forces that can directly affect cells are described below.
(1) “Compression” is a pressing force that can physically provide external pressure on plasma membranes or sub-organelles. (2) “Tension” is a pulling force that causes coercive expansion of the cell structure. (3) “Shear force” is associated with the amputation of a cellular organism that results in cell structures pushing each other in opposite directions. (4) “Torque” is a force that induces twisting and pinching stimulation toward cellular membranes or sub-organelles. Such forces can be exerted on cells through the various synchronized physical motions of the mechanical stimuli-activated functional materials, and consequently, these mechanical stimulations can exert a direct anti-cancer therapeutic effect by inducing cell structural collapse and thus cellular damage. Ultrasound and magnetic fields can generate mechanical forces on cell structures such as the plasma membrane surface or internal sub-organelles (e.g., mitochondria, lysosomes, or nuclei) that induce coercive morphological changes and intracellular damage. Furthermore, it can provide indirect support to enhance other treatment systems such as chemotherapy, immunotherapy, gene therapy, gas therapy, PDT, and sonodynamic therapy (SDT) as combinational therapeutic strategies. Hence, mechanical cancer therapy achieved by utilizing ultrasound or magnetic fields represents a promising modality that offers effective cancer treatment through the simple application of versatile mechanical stimulation (Scheme 1).
Scheme 1 Mechanical stimuli-driven cancer therapeutics mediated by various mechanical forces (compression, tension, shear force, and torque) via the application of ultrasound and magnetic fields. |
This tutorial review describes the fundamental principles of the assorted types of mechanical forces induced by ultrasound- and magnetic field-driven stimuli and their interactions with target sub-organelles in microscopic cellular environments. Additionally, various therapeutic strategies for cancer therapy are summarized in addition to mechanical stimulation using ultrasound and magnetic fields. We will discuss novel functional materials, their characterization, and their biological applications, although it should be noted that these still need further consideration for real clinical applications (Table 1).
Energy sources | Mechanical forces | Nano/micro materials | Force-generating mechanism | Irradiation modes | Therapeutic application | Ref. |
---|---|---|---|---|---|---|
Abbreviations: NPs, nanoparticles; FU, focused ultrasound; DC, duty cycle; MB, microbubble; US, ultrasound; TSP, temperature-sensitive plateletsome; HIFU, high-intensity focused ultrasound; siRNA, small interfering ribonucleic acid; HMPB, hollow mesoporous Prussian blue; PFH, perfluorohexane; tHSA, thiolated human serum albumin; PLGA, poly(lactic-co-gycolic acid); LPO, lipid peroxidation; MOF, metal–organic framework; MNPs, magnetic nanoparticles; SMF, static magnetic field; SPIONs, superparamagnetic iron oxide nanoparticles; PMF, pulsed magnetic field; RMF, rotating magnetic field; AMF, alternating magnetic field; HAFP, hyaluronic acid, protamine, and ferumoxytol; MNM, magnetic nanomotor; IONs, iron oxide nanocubes; USPIONs, ultra-small superparamagnetic iron oxide nanoparticles; SMNPs, superparamagnetic nanoparticles; DMF, dynamic magnetic field. | ||||||
Ultrasound | Compression + Shear force | Liposomal NPs | Sonoporation | FU (3 W cm−2, 1 MHz, 10% DC) | Neuroblastoma | 30 |
MB-NPs | Sonoporation | US (4 W cm−2, 1 MHz, 50% DC) | Breast cancer (4T1) | 31 | ||
Spherical siRNA-NPs | Sonoporation | FU (10 W cm−2, 1.5 MHz, 5% DC) | Melanoma (B16F10), carcinoma (SCC-7) | 32 | ||
Tension | TSP | MB implosion | HIFU (0–200 W, 0.6–1.8 MHz) | Carcinoma (HeLa) | 34 | |
HMPBs-PFH | MB implosion | HIFU (120 W, 5–12 MHz) | Breast cancer (MDA-MB-231) | 35 | ||
Spherical tHSA-NPs-MB | MB implosion | HIFU (10 W cm−2, 1.5 MHz, 5% DC) | Lung cancer (A549) | 36 | ||
Fe3O4-PFH/PLGA | MB implosion | HIFU (0.8 MHz) | Carcinoma (VX2) | 37 | ||
Shear force | Liposome | LPO | US (1 W cm−2, 1 MHz, 50% DC) | Breast cancer (4T1) | 45 | |
Liposome | LPO | US (0.35 W cm−2, 1 MHz) | Breast cancer (4T1) | 46 | ||
Ga-Fe(II)@liposome | LPO | US (1.5 W cm−2, 1 MHz, 50% DC) | Breast cancer (MCF-7/ADR) | 47 | ||
Mn-MOF | LPO | US (0.9 W cm−2, 1 MHz, 30% DC) | Breast cancer (4T1) | 48 | ||
Magnetic field | Compression + Tension | Spherical MNPs (Zn0.4Fe2.6O4) | Directional MNP movement | SMF (0.2 T) | Colon cancer (DLD-1) | 59 |
Spherical SPIONs (Fe3O4) | Pulsed SPION movement | PMF (5–8 T, 10 pulses, 10 s interval) | Liver cancer (Huh7, Alexander, and HepG2) | 61 | ||
Sharp MMP (Fe3O4) | MMP vibration | RMF (0.6 T, 20 Hz) | Glioblastoma (U87-MG) | 69 | ||
Spherical MNPs (Fe) | MNP vibration | OMF (0.2 T, 10 Hz) | Liver cancer (HepG2) | 76 | ||
Spherical HAPF | HAPF vibration | AMF (1.5 mT, 1 Hz) | Natural killer cell | 77 | ||
Spherical MNPs (Zn0.4Fe2.6O4) | Directional MNP movement | SMF (0.5 T) | Drug-resistant colon cancer (DLD-1/ADR) | 78 | ||
Shear force | Rod-like MNPs (Zn0.4Fe2.6O4) | MNP rotation | RMF (40 mT, 15 Hz) | Glioblastoma cell mitochondria (U87), liver cancer cell mitochondria (HepG2) | 71 | |
Rod-like FDP (Fe3O4@Dex-PGEA) | FDP rotation | AMF | Breast cancer (4T1) | 80 | ||
Rod-like MNM (Zn0.4Fe2.6O4) | MNM rotation | RMF (40 mT, 15 Hz) | Glioblastoma (U87), breast cancer (MDA-MB-231) | 81 | ||
Rod-like IONs (Zn0.4Fe2.6O4) | ION rotation | RMF (40 mT, 15 Hz) | Glioblastoma (U87) | 82 | ||
Torque | Microdisc (FeNi) | Microdisc spin | AMF (9 mT, 10–20 Hz) | Glioma (N10) | 54 | |
Spherical MNPs (Fe3O4) | MNP spin | AMF (50 mT, 233 kHz) | Breast cancer (MDA-MB-231) | 63 | ||
Spherical USPIONs (Fe3O4) | USPION rotation | RMF (40 mT, 1 Hz) | Pancreatic cancer-associated fibroblast lysosome (CAF-CCK2) | 64 | ||
Spherical SMNPs (Fe3O4) | SMNP spin | AMF (125 mT, 50 Hz) | Breast cancer (MDA-MB-231, and BT474) | 66 | ||
Spherical USPIONs (Fe3O4) | USPION rotation | RMF (40 mT, 1 Hz) | Pancreatic cancer-associated fibroblast lysosome (CAF-CCK2) | 74 | ||
Spherical SPIONs (Fe3O4) | SPION spin | DMF (30 mT, 20 Hz) | Insulinoma cell lysosome (INS-1) | 84 | ||
Nanospindle (Fe3O4) | Nanospindle spin | AMF (1 mT, 0.5–20.0 Hz) | Hepatoma cell mitochondria (McA-RH7777) | 85 |
The ultrasound-driven mechanical forces that arise from cavitation are related primarily to bubbles that are exogenously administered or created during the rarefactional cycle of acoustic pressure.20–23 The gas in a solution of body tissues is converted into microbubbles (MBs) through negative pressure or the administration of stabilized gas-filled MBs or nanobubbles (NBs) in factitial biomaterials.24 The bubbles then oscillate, burst, and impact nearby cellular building blocks such as cell membranes or sub-organelles.25 Hence, ultrasound-mediated mechanical cancer therapy with low-frequency insonation in the cancer region should be considered a practical cancer therapeutic tool in the clinical field, and many other versatile approaches have been suggested according to significant research.
It should be emphasized that ultrasound-mediated mechanical stimulation is associated with acoustic cavitation bubble collapse close to or on a solid cellular structure. The solid surfaces provide resistance to liquid flow above a given pressure threshold, and bubble implosion occurs beyond a certain resonance size. Resultantly, the bubbles collapse asymmetrically,21 which produces a high-speed liquid jet (micro-jetting) that moves at a speed as fast as 111 m s−1 on the cellular structure.25,26 Hence, severe damages can be inflicted on the impact zones and surface pitting (erosion) can occur on the surface. Moreover, the local environment of a plasma membrane is influenced by acoustic streaming, and this is called microstreaming (fluid streamlines).23,24 Due to the pressure differences, microstreaming is generated around the MBs as they undergo vibrational motions, ultimately driving the surrounding fluid to flow around it in several patterns.24 Consequently, microstreaming could physically interfere with the interaction of the nearby lipid membranes. Both micro-jetting and microstreaming contribute to weakening the robustness of the cell membrane, and this induces “compression” and “shear forces.” Another fundamental physiological property of acoustic cavitation is sonoluminescence which refers to light production during insonation.24–26 Although the exact mechanism underlying light production in sonoluminescence is still unclear, it has been suggested that blackbody, bremsstrahlung, or recombination radiation, alone or in combination, may result in sonoluminescence. A sonosensitizer can be excited by sonoluminescence to generate electron–hole (e−–h+) pairs or triplet states with subsequent generation of reactive oxygen species (ROS) or radical species (˙OH) that react with endogenous substrates,27,28 particularly lipid layers as a lipid peroxidation (LPO), which can induce “shear force.”
Consequentially, these ultrasound-induced mechanical stimuli yield inertial cavitation, where inward rushing of fluid against unstable and expanding bubbles causes micro-jetting, bubble collapse, and implosion. Eventually, insonated cavitation results in energy release in the form of a shockwave yielding transient sonochemical hotspots (up to 10000 K) that correspond to a liquid layer, high pressures (81 MPa), and mechanical forces. An ultrasound-mediated mechanical stimuli can generate the following forces.
(1) Bubble oscillations: sono-compression and tension caused by implosion or rapid shrinkage of MBs on the exterior plasma membrane. (2) Fluid streamlines: sono-compression induced by microstreaming tears the lipid layer apart from the vibration MBs outside the plasma membrane. (3) Cavitation events: sono-compression and shear force generated by micro-jetting or sono-tension induced by pulsatile cell swelling with HIFU. (4) LPO: induced shear force generated by peroxidation of the lipid membrane structure. These mechanical forces not only directly damage cancer cells to induce apoptotic or non-apoptotic cellular death but also induce indirect therapeutic mediation by enhancing other anti-cancer treatment systems (Fig. 1b).
Fig. 2 Sono-compression and shear force-mediated cancer therapy. (a) Schematic illustration of sono-compression and shear force-mediated sonoporation to enhance anti-cancer therapeutic strategies under ultrasound irradiation. (b) Graphical illustration of combining sonoporation to enhance the chemotherapeutic effect with liposomal doxorubicin (L-DOX). Reproduced with permission from ref. 30. Copyright, 2020 Ivyspring International Publisher. (c) Schematic representation of nano-complex-decorated MBs targeting CD11b on antigen-presenting cells (APCs) under ultrasound exposure to activate synthase-stimulator of interferon genes (STING) and downstream antitumor immunity. Reproduced with permission from ref. 31. Copyright, 2022, Nature publishing group. (d) Schematic illustration of ultrasound-responsive nanocomplex composed of siRNA-NP and PTX-MBs under the sonopermeable effect for enhanced gene and chemotherapy. Reproduced with permission from ref. 32. Copyright, 2020, Elsevier. |
In 2020, Sirsi's group reported MB-assisted ultrasound imaging for monitoring the effects of sonoporation and improving the efficacy of liposomal drug uptake in the context of neuroblastoma.30 Currently, most cancer-targeting nanomedicines focus on passive accumulation as an enhanced permeability and retention (EPR) effect. To overcome the reliance on the EPR effect, a combination of ultrasound and therapeutics has been utilized. This strategy was first introduced based on liposomal doxorubicin (L-DOX, or DoxilTM) which is an FDA-approved nanomedicine and results in an effective delivery using FU and MBs by compression- and shear force-mediated sonoporation without thermal damaging of the tumor vascular network that is essential for continuous chemotherapy delivery to tumors beyond the brain (Fig. 2b).30
Lux et al. developed nanocomplex-conjugated antigen-presenting cell (APC)-targeting MBs for new immunotherapeutic targeting regulators of the innate immune system in cancer. Ultrasound-mediated sonoporation demonstrated a targeted activation synthase-stimulator of interferon genes (STING) platform as a robust strategy for MB-assisted ultrasound-guided immunotherapy of cancer (MUSIC).31 The STING pathway is essential for innate and adaptive immune responses through APCs in cancer. To date, STING antagonists have raised concerns regarding the limit of entry into the cytoplasm and systemic toxicity. However, they proposed a combination of MB-mediated ultrasound and a method to overcome these limitations. The MUSIC platform was the first image-guided cancer immunotherapy that enhanced STING activation following compression- and shear force-induced sonoporation in APCs via drug delivery using specific antibody targets (Fig. 2c). Image-guided immunotherapy using ultrasound-responsive biomaterials generates robust antitumor effects while minimizing systemic toxicity through efficient and targeted potent immune activation for application in various cancers.
Among nanomedicine combination therapies, chemo-gene therapy is emerging as an approach for enhanced anti-cancer efficacy due to its synergistic effects and reduced chemotherapy doses. Nevertheless, effective co-delivery of drugs and therapeutic genes and drugs into targeted cells and tissues is still a limitation. The extracellular matrix (ECM), one of the components of the tumor microenvironment, also inhibits the effects of chemo-gene therapeutics by preventing chemical gene therapy through the infiltration of blood vessels into the cell layers of the tumor. To overcome the limitations of chemo-gene combination therapy, Kim's group has applied an FU-responsive nanocomplex that is composed of siRNA-nanoparticles and paclitaxel-loaded MBs (PTX-MBs) for enhanced chemo-gene therapeutics.32 FU plays an essential role as a non-invasive, safe, and potential therapeutic method. MBs induce the sonoporation effect by FU exposure and loosen the ECM structure via a strong mechanical effect with the nanocomplex for improved penetration of chemo-gene therapeutics (Fig. 2d). As a result of the enhanced FU-mediated dual delivery system, a dramatic antitumor effect was demonstrated by the sonoporation. These strategies are also effective for use in other cancers and are therefore potential therapeutics for controlling dosing and predicting outcomes in clinical applications.
Fig. 3 Sono-tension-mediated cancer therapy. (a) Schematic illustration of tension and hyperthermia cytotoxic effect by utilizing high-intensity focused ultrasound (HIFU) irradiation. (b) Graphical representation and transmission electron microscopy (TEM) images of DTSP-3 and DTSP-4 after non-treatment, water bath heating (42 °C, 2 min), and HIFU-irradiation that indicate the HIFU-based cellular tension and hyperthermia damage to cancer cells. The scale bar represents 100 nm. Reproduced with permission from ref. 34. Copyright, 2019, Ivyspring International Publisher. (c) Graphical representation of enhanced cellular uptake of Cy5.5-DiI-tHSA-NPs-MBs and tension-mediated damage with either ultrasound or HIFU exposure. Green and red fluorescence signals indicate DiI and Cy5.5, respectively. Reproduced with permission from the ref. 36. Copyright, 2017, Elsevier. |
Jing et al. constructed nano-vesicular temperature-sensitive platelesomes that mimic liposomes by integrating 1,2-dipalmitoyl-sn-glycero-3-phosphocholine, 1-stearoyl-2-hydroxy-sn-glycero-3-phosphocholine, and platelets membrane.34 TSPs can provide higher tumor drug utilization during HIFU thermotherapy in combination with HIFU ablation to provide effective chemotherapy. Most importantly, after HIFU ablation, TSP can spontaneously localize to the postoperative tumor site by adhering to the damaged tumor vessels, thus resulting in ideal consolidation chemotherapy (Fig. 3b). Furthermore, Zheng's group designed a multifunctional therapeutic nanoplatform with HMPBs encapsulating DOX and PFH to achieve in vivo synergistic effects of HIFU/chemotherapy.35 When stimulated by HIFU, PFH bubbles released through mesoporous channels in the HMPB shell can alter the acoustic environment and induce cavitation effects, thereby improving the performance of ultrasound imaging and HIFU therapy.
Han et al. demonstrated HIFU-mediated mechanical stimulation and DDS that was composed of PTX-tHSA-NPs and MBs to achieve enhanced therapeutic efficacy.36 HIFU exposure exhibited superior chemotherapeutic efficacy due to the greater cavitation effect of HIFU in conjunction with MBs. Additionally, HIFU may induce significant thermal- and tension-mediated mechanical damage under the HIFU stimuli (Fig. 3c). Li's group developed a multifunctional nanoparticle-based Fe3O4-PFH/PLGA for HI FU synergistic tumor surgery.37 Due to the integration of superparamagnetic Fe3O4 nanoparticles within the shell and the phase change properties of the PFH core, these nanocapsules were further developed as contrast agents for ultrasound, MR, and PA trimodal imaging and were also used as synergistic agents for enhanced HIFU ablation. In contrast, the combination of transarterial chemoembolization and nanoparticle-enhanced HIFU surgery can effectively improve the treatment outcome of liver cancer, thus providing a new cancer treatment option.
Fig. 4 Sono-shear force-mediated cancer therapy. (a) Schematic illustration of ultrasound stimuli-induced lipid peroxidation (LPO) of long-chain polyunsaturated lipid structure as an induced shear force on the plasma membrane for cancer therapy. (b) Graphical representation of the synergistic therapeutic mechanism of sonodynamic therapy (SDT)-based ferroptosis targeting. The targeted delivery of iron oxide nanoparticles ferumoxytol exhibits a ferroptosis-inducing function by importing iron ions and SDT for LPO-induced membrane shear force, thus promoting cellular-selective autophagy. Reproduced with permission from ref. 45. Copyright, 2021, Elsevier. (c) Schematic illustration of induced shear force by utilizing sonodynamic amplified ferroptosis erythrocyte (SAFE) that was composed of an iRGD peptide and red blood cell membrane (RBCM) hybrid camouflaged sono-responsive therapeutic nanocomplex of hemoglobin (Hb), perfluorocarbon (PFC), ferroptosis activator (dihomo-γ-linolenic acid, DGLA), and sonosensitizer (verteporfin, Vp) for the combination treatment of SDT and ferroptosis under ultrasound stimulation. Reproduced with permission from ref. 46. Copyright, 2022, John Wiley & Sons. |
In 2021, Chen et al. demonstrated synergistic tumor treatment via an SDT-based LPO mechanism to induce ferroptosis using ferumoxytol and a PpIX sonosensitizer (Fig. 4b).45 As a result, surged unstable Fe2+ in tumor cells caused cell death mediated by ROS-induced ferroptosis. Unlike other typical mechanisms of cell death, such as apoptosis, necrosis, and autophagy, ferroptosis causes substantial damage to mitochondrial shapes such as volume shrinkage and increased membrane density. Furthermore, endogenous biological metabolites have been identified as potential ferroptosis-inducing agents that produce intracellular lipid peroxides, thereby increasing cell vulnerability to ferroptosis. Dihomo-linolenic acid (DGLA) is generated in vivo from the essential fatty acid linolenic acid and functions as a ferroptosis substrate that can be oxidized by ROS in irreversible intracellular LPO.
Regarding the mechanism of shear-force-driven deformation of lipid membranes, vesicles adapt to the applied shear force by creating stress in the membrane and deforming it into an elliptical shape. The shear force-stressed cells appeared to exhibit orientation and extension along the flow direction. Zhou et al. reported sonodynamic-amplified ferroptosis erythrocytes (SAFEs) that exhibited sonodynamic peroxidation of lipids for improving ferroptosis therapy (Fig. 4c).46 In particular, ultrasound-targeted MB is an effective method that combines low-intensity ultrasound with MBs (UTMD). UTMD medication delivery is a targeted technique compared to standard systemic chemotherapeutic drug administration. When external ultrasound irradiation is focused on a tumor, only MBs passing through the beam interact with the ultrasound energy, thus allowing targeted medicine delivery.
In 2022, Zheng and co-workers reported an ultrasound-mediated catalytic and chemotherapeutic liposomal nanomedicine by encapsulating an effective iron-based Fenton catalyst GA-Fe(II) and DOX for synergistically augmenting DOX-resistant breast cancer cells (MCF-7/ADR).47 It exhibited a significant therapeutic effect via a complementary ferroptosis/apoptosis induction modality. GA–Fe(II) complexes can constantly catalyze ˙OH production that is attributed to the depletion of GSH, accumulation of intracellular iron, and enhanced generation of induced-shear force by LPO to actively trigger the iron-dependent ferroptosis pathway within tumor cells. Similarly, Gan et al. demonstrated a versatile manganese porphyrin-based metal–organic framework (Mn-MOF) nanoplatform for enhanced SDT and LPO-based ferroptosis.48 Mn-MOF exhibited high catalytic activity that catalyzes tumor overexpressed H2O2 to generate O2 for tumor hypoxia relief. Meanwhile, Mn-MOF decreased intracellular GSH concentration and GPX4 activity, and this led to enhanced SDT and LPO to achieve significant induced-shear force and cancer cell ferroptosis.
The magnetic field has been utilized to modulate the movement of MNMs in various shapes (e.g., nanospheres, nanorods, nanohelices, nanodiscs, etc.) for cancer therapeutics, tissue regeneration, immunoregulation, and diagnostic biomedical applications52–57 due to their high tissue-penetrative, biocompatible, and spatiotemporally controllable characteristics.58,59 In most cases, MNMs are chemically functionalized with drugs and/or ligands to precisely deliver therapeutic agents, modulate cellular responses, or increase imaging contrast for enhanced efficiency. In general, various mechanical motions of the MNMs can be controlled by changing the applied magnetic field modes, where a static magnetic field (SMF) induces directional movement, a pulsed magnetic field (PMF) induces discrete directional movement, an oscillating magnetic field (OMF) induces periodic movement (oscillation), an alternating magnetic field (AMF) induces spin and vibration, and a rotating magnetic field (RMF) induces rotation about a point and vibration (Fig. 5a).60–64 The AMF flips the magnetic field direction while the RMF rotates the magnetic field direction. Hence, magnetic field modes can be selected for the required mechanical motion to enable precise cancer therapy.65
In general, MNM shape-dependent motion is tunable with highly anisotropic MNMs exhibiting the shape of the nanorods, nanohelices, nanodiscs, and others that have been actively programmed for microrobot applications.49,50,67,68 They exhibit preferential magnetization along the length axis over the diameter axis in the cases of “nanorod” and “nanohelix” shapes and along the basal plane over out of plane in the case of “nanodisc” shape. Therefore, selectively controlling both the shape of the MNMs and the applied magnetic fields can modulate the dominant types of mechanical forces exerted on cells. Compression and tension can be vertically exerted on cell membranes or organelles by the MNMs in any shape, including magnetic nanospheres under the SMF and PMF, while the MNMs in sharper shape can apply higher compression on cells that are more cell-penetrative.60,61,69 The rubbing effect (shear force and compression) can be laterally applied to cell membranes or organelles by the MNMs under the OMF, AMF, and RMF.70 The spin and rotation about a point of the MNMs arise from the rotation of their magnetic spin and moment (μ) to directionally align themselves with the changing external magnetic field (B).65 Typically, the AMF induces their spinning that possibly applies torque to cells, and the RMF induces their rotation about a point that possibly applies a shear force to cells. Shear force and localized compression can be exerted to “slice” cells by the propeller-like motion of “magnetic nanorods” due to preferential rotation of magnetic spins that are easily aligned along the length axis under the AMF.55,71 Shear forces can be spirally applied to “mill” cells by the spinning of the “magnetic nanohelices” around their major axis due to preferential rotation and translation of magnetic spins that are easily aligned along the curved direction of nanohelices under the RMF.72,73 Shear force and extensive compression have been applied to “scrap” cells by the rotation of the “magnetic nanodiscs” possessing flat morphology due to preferential rotation of magnetic spins that are readily aligned along the basal plane under the AMF.54
Furthermore, the shape-specific mechanical effect exerted on cells can be synergized with the chemical functionalization of MNMs to target or bind to specific cell membranes or organelles. When the MNMs are bound and confined to the targeted organelles, the motion of the MNMs can directly exert mechanical forces on cells such as inflicting localized compression and tension (vibration) under the OMF, localized torque under the AMF, or localized shear force under the RMF.66,74 Additionally, depending on the magnitude of the imposed forces and targeted organelles, such forces can damage cellular structures or stimulate mechanosensitive receptors and ion channels for therapeutic purposes.75 Specifically, forces exerted on cell organelles temporarily deform them until the forces surpass their tolerance to disrupt them. Typically, a pN scale of force is required for the modulation of cells such as several tens to hundreds of pN to rupture cell membranes, 500 pN to disrupt lysosomal membranes, 320 pN of torque or 600 pN of tension to break the actin–actin bonds, and several pN to activate mechanosensitive ion channels.61,64,66,74 Ultimately, these forces result in cell apoptosis, necrosis, or activation to induce cancer cell apoptosis, necrosis, or immune cell activation for cancer immunotherapy. Therefore, the specific desired mechanical motion of the MNMs can be precisely tailored with versatile tuning of their shape, functionalization, and applied magnetic field modes, and they can offer novel modalities for magnetic field-mediated mechanical cancer therapy.
Fig. 6 Magnetic compression- and tension-mediated cancer therapy. (a) Schematic illustration of magnetic nanosphere-induced cancer cell apoptosis through the compression and tension exerted on the cell membrane under an alternating magnetic field (AMF). (b) Graphical illustration of NK cell activation through vibration (compression and tension) imposed on the cell membrane under the AMF. Reproduced with permission from ref. 77. Copyright, 2021, American Chemical Society. (c) Schematic representation and immunofluorescence images of cancer cell apoptosis through lysosome permeabilization using magnetic nanosphere-induced tension on the lysosomes of cancer cells under the pulsed magnetic field (PMF). Scale bars: 50 μm. Reproduced with permission from ref. 61. Copyright, 2019, MDPI. (d) Schematic illustration and TEM images of tension acting on the cancer cell membrane by pulling the magnetic nanosphere-bound receptors using a static magnetic field to induce apoptotic signals in cancer cells. Scale bars: 1 μm. Reproduced with permission from ref. 78. Copyright, 2016, American Chemical Society. |
For example, it was recently reported that sharp magnetic materials can induce cancer cell necrosis in its bare state or apoptosis in its functionalized form under RMF.69 As the materials were adsorbed on the cell membrane and internalized, their vibration via RMF (2 Hz, 600 mT) imposed localized compression and tension on the cell membrane to decrease cell membrane integrity, ultimately resulting in lactate dehydrogenase leakage as high as 96%. When the reported material was functionalized with poly(ethylene glycol) (PEG), the PEG chains provided steric hindrance to mitigate direct cell-damaging effects caused by the sharp edges. Bossis's group reported the necrosis of cancer cells via compression in vibration when treated with magnetic nanospheres under OMF (10 Hz, 200 mT).76 In detail, liver carcinoma cells were supplemented with a cell culture medium containing the spindle-like particles (1% mass ratio) and subjected to the vertically oscillating magnetic field. Simple indentations made by particle compression are not sufficient to cause membrane damage. However, the aggregation of the particles observed upon oscillating magnetic field exposure generated pressure caused by particle clustering that physically damaged the cell membrane to induce necrosis. These results confirm that the structure of MNMs can alter the cell-damaging mechanism and must be carefully selected to precisely modulate their therapeutic effect.
In 2021, specifically targeted mechanical vibration was introduced for cancer immunotherapy (Fig. 6b).77 Magnetic nanocomplexes were composed of hyaluronic acid and protamine to allow NK cell labeling and ferumoxytol to endow magnetic controllability. The application of AMF (1 Hz, 1.5 mT) vibrated the nanocomplexes in compression and tension, which stimulated mechanosensitive structures of NK cells such as actin and natural killer group receptors, thereby upregulating their cytolytic activity. Practical in vivo application of magnetic NK cell activation in a rat tumor model confirmed its therapeutic efficiency by suppressing the tumor growth rate, thus suggesting the potential of exerting vibration (compression and tension) for cancer immunotherapy. Tension was also introduced on the lysosomes of cancer cells for their disruption.61 In this research, magnetic nanospheres were loaded into the lysosomes of cancer cells through endocytosis and subjected to the PMF of high strength (8 T) and short pulse width (15 μs) (Fig. 6c). The nanospheres were pulled towards the external magnetic field in a pulsed manner, thereby imposing pulsed tension on the lysosomal membrane. Ultimately, the cell membrane was permeabilized, and cathepsin B (protease) leaked into the cytoplasm to induce cancer cell apoptosis. Cheon's group applied tension to facilitate cell apoptosis by magnetically activating death receptor 4 on cancer cells (Fig. 6d).59 The magnetic nanospheres bind to the receptor via an antibody reaction, and the SMF (200 mT) was applied to pull (applying tension) and cluster the magnetic nanosphere-bound receptors without disrupting other cytoskeletal structures. Notably, apoptosis signaling was proportional to SMF strength, with increasing strength causing more receptors to be pulled together in vivo. Subsequently, a death-inducing signaling complex was formed and caspase-3 was activated, ultimately resulting in cancer apoptosis. This approach was also effective in treating multidrug-resistant cancers, thus supporting the clinical applicability of applying tension in cancer therapy.78
Fig. 7 Magnetic shear force-mediated cancer therapy. (a) Schematic illustration of magnetic nanorod-induced cancer cell destruction through shear force exerted on the cell membrane under a rotating magnetic field (RMF). (b–d) Cancer cell apoptosis triggered by shear force generated by the rotation of rod-like magnetic nanocube assembly under RMF exposure. (b) Schematic illustration of shear force-activated ROS production. Reproduced with permission from ref. 82. Copyright, 2020, John Wiley & Sons. (c) Immunofluorescence images and scanning electron microscope (SEM) images illustrating the morphological change of cancer cells after shear force-mediated cancer therapy under RMFs at various frequencies (control, 5, 10, and 15 Hz). The cytoskeleton and nucleus were stained with phalloidin and DAPI, respectively. Scale bars: 20 mm (immunofluorescence) and 5 μm (SEM). Reproduced with permission from ref. 81. Copyright, 2021, American Chemical Society. (d) Schematic illustration and time-dependent images of cancer cell destruction by mitochondrial-targeted RMF treatment. Scale bar: 20 μm. Reproduced with permission from ref. 71. Copyright, 2020, John Wiley & Sons. |
In 2021, the rotational motion of rod-shaped MNMs was reported to inflict shear forces on cancer cells to induce apoptosis more efficiently than that induced by spherical MNMs.80 The viability of cancer cells after AMF treatment was significantly lower for those that internalized rod-shaped MNMs compared to that of spherical MNMs. Moreover, the morphological deformation of cancer cells increased when subjected to rod-shaped MNM rotation exerting a shear force, thus demonstrating the MNM shape-dependent effect of mechanical forces exerted on cells. Translation of this rotational shear force in vivo for the treatment of breast tumor-bearing mice resulted in a marked decrease in the relative tumor volume compared to that of the non-treated group. Recently, Cheng's group reported a novel mechanism for cancer cell destruction using magnetic nanocubes that assemble into rod-like morphology and rotate (shear force) upon RMF (15 Hz, 40 mT) exposure (Fig. 7b and c).71,81,82 The nanocubes were designed to be either simply internalized or to target mitochondria for effective cancer therapy. With their interparticle dipolar interactions and geometrical flat surface, exposure to the RMF helped to assemble the magnetic nanocubes into a rod-like morphology. Continuous subjection to the RMF then induces the spinning motion of rod-like nanocube assembly that inflicts a slicing effect (shear force) on cellular organelles.81 The MNMs that do not target specific organelles destroyed the cell membrane and other subcellular structures, thereby activating caspase-3 and elevating ROS production.82 Material functionalization to target mitochondria allowed the localized shear force to be directly imposed upon the mitochondrial membrane, thus resulting in pro-apoptotic proteins such as cytochrome c being leaked into the cytosol (Fig. 7d).71
Fig. 8 Magnetic torque-mediated cancer therapy. (a) Schematic illustration of magnetic microdisc-induced cancer cell apoptosis by the torque exerted on the cell membrane under an AMF. (b) Graphical representation of the lysosomal membrane rupture through magnetic nanosphere rotation under the AMF to induce cancer cell apoptosis and fluorescence images of lysosomes (LysoTracker Green) in cancer cells in the presence and absence of targeted torque. Scale bars: 5 μm. Reproduced with permission from ref. 84. Copyright, 2014, American Chemical Society. (c) Schematic illustration of the mitochondria damage induced by the torque generated through magnetic nanospindle rotation under the AMF and in vivo T2 magnetic resonance images of pre- and post-injection of MNMs. Reproduced with permission from ref. 85. Copyright, 2021, Royal Society of Chemistry. |
In 2022, Carrey et al. reported RMF (1 Hz, 40 mT) treatment of functionalized magnetic nanospheres internalized in cancer cells for tumor therapy.64,74 Specifically, the torque created by nanosphere rotation (3 pN) was not sufficient to directly damage the lysosomal membrane. However, its cumulative exposure was sufficient to disrupt lysosomal integrity, activate ion channels, and induce lysosomal movement. As a result, the targeted cancer cells undergo apoptosis due to lysosomal leakage and microtubule disruption.
To exert torque on cell organelles more efficiently, MNMs were bound to them before they were subjected to AMF. For example, the spinning (torque) of magnetic nanospheres is stimulated when they are bound to lysosomes (membrane twisting) under AMF (20 Hz, 30 mT) (Fig. 8b).63,84 The magnetic nanospheres were functionalized to bind to the lysosomal membrane receptors so that the spinning motion of nanospheres could be directly transmitted to the membrane as twisting energy (torque). As the generated force exceeds membrane tolerance, the membrane tears apart, thus resulting in cancer apoptosis and impaired cancer growth. Master et al. tethered magnetic nanospheres to the lysosomal membranes via polymer network formation to further enhance the effect of torque on cancer cell apoptosis.66 To elaborate, the magnetic nanospheres were coated with polymers to be internalized into cancer cells and anchored to the lysosomal membrane to induce cancer cell cytoskeleton rupture using robust polymer network formation of the MNMs with AMF (50 Hz, 125 mT) for cancer cell destruction. This resulted in the spinning force (torque) of lysosomes being directed to the points of attachment with actin filaments to rupture the cytoskeleton and trigger cancer cell death in response to the external AMF. Different functionalization of MNMs can alter cancer therapeutic mechanisms and effects.
In 2021, Kim's group reported mechanical induction of mitophagy by directly imposing torque on mitochondria (Fig. 8c).85 In detail, magnetic nanospindles were modified with ligands to bind to mitochondria and then subjected to the AMF (0.5–20.0 Hz, 1 mT) that triggered the NMMs to spin (torque) on the mitochondrial surface and damage mitochondrial membranes. Shape-specific mechanical effects contribute to mitochondrial dysfunction. This results in excessive mitophagy that eventually leads to cancer cell death. The spinning of magnetic microdiscs was also introduced to trigger cancer apoptosis by initiating programmed cancer death54 in which the materials were coated with antibodies to attach to the cancer cell membrane and then exposed to AMF (10–20 Hz, 9 mT). Although the twisting force (torque) generated by microdisc spinning was insufficient to rupture the membrane, it triggered apoptosis-related receptors and ion channels. The scrapping effect generated by the spinning of flat and wide disc shapes was considered to activate programmed cell death. Cellular analysis of its morphology and inner ion levels revealed that disturbance of calcium homeostasis via calcium channel perturbation resulted in 90% cancer cell apoptosis.
Nevertheless, further progress in the advanced therapeutic applications of ultrasound-mediated cancer therapy requires a better understanding of the physical fundamentals of molecular events and their interactions with tissue-related biological effects. For example, the described cavitation motion of MBs is not well defined. Therefore, it must be precisely characterized to identify possible mechanical forces (e.g., compression, tension, or shear force) responsible for the observed effects. Moreover, it is unknown if ultrasound-driven physical stimulation can substantially exert a significant impact on the cellular structure, and this may provide robust evidence for future ultrasound-mediated mechanical cancer therapy research. Additionally, at lower levels of exposure to ultrasound, there is no established evidence of any specific harmful effects; however, very little research data are available to draw firm conclusions, particularly concerning the long-term use of ultrasound. These subtle effects are not yet entirely understood.
Despite advances in ultrasound, including various physical properties and versatile interaction modes in a biological system, except for imaging techniques this technique has not yet been at the center of clinical applications. Therefore, the development of advanced materials for sonoporation-based DDS, novle sonosensitizers, and techniques of ultrasound machines headed by HIFU make ultrasound-mediated mechanical cancer therapy a promising component in an effort to improve cancer treatment outcomes. Additionally, synergistic combination therapy with conventional anti-cancer therapy provides a promising strategy to compensate for its limitations.
Despite such unprecedented clinical potential, understanding how mechanical forces are exerted on cells is critical for the successful translation of magnetic-field-based cancer therapy. The MNM shape- and functionalization-dependent motion must be further designed and precisely characterized to identify the diverse mechanical forces (e.g., compression, tension, shear force, and torque) imposed on cancer cell membranes and organelles. Calculation of the magnitude of the forces imposed on cells is pivotal for estimating if such forces damage cellular structures or stimulate mechanosensitive receptors and ion channels in cancer cells. Targeting cancer cells must be maximized via MNM functionalization to ensure the safe application of magnetic-field-based cancer therapy.
Magnetic-field-based mechanical cancer therapy can overcome the drawbacks of conventional therapies such as drug tolerance and off-target effects. The versatility of MNMs presents unique opportunities for synergistic combinatorial and imaging-guided therapies. For example, the degradation of MNMs can supply iron ions to trigger the Fenton reaction, thereby enabling chemodynamic therapy in synergy with magnetic field-based mechanical cancer therapy.89 The MNMs can also label cells to enable magnetic resonance imaging-guided mechanical cancer therapy.77 The motion of the MNMs can exert mechanical stimuli to cell membranes to activate ion channels and thus trigger cancer cell apoptosis.54 These combinatorial therapies and imaging-guided therapies coupled with the chemical synthesis of novel MNMs can present unprecedented clinical utilities for magnetic field-based mechanical cancer therapy.
Footnote |
† These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2023 |